China is the country with the largest emissions of SO2 and mercury in the world, with about 75% of the SO2 emission and 65% of the mercury emission resulting from coal combustion. As the pollution situation caused by coal combustion is very serious, China is also very concerned about mercury-based heavy metal pollution prevention in addition to focusing on managing coal-fired SO2, NOx and particulate pollution. A single technology is mostly adopted for controlling different pollutants at home and abroad, for example, the flue gas mercury removal technology developed by the National Energy Technology Laboratory (NETL) of the United States has a mercury removal rate of up to about 90% due to the use of activated carbon injection and dust removal technologies, however, industrialized popularization and application are difficult for this technology due to its high cost of investment, high cost of activated carbon and great difficulty in eluting activated carbon. Furthermore, it may result in issues such as mismatching among various equipment, high cost of comprehensive investment, large covering area, high operation cost and inconvenient maintenance because different management equipment is used for controlling different pollutants.
Concerning the issues stated above, domestic and foreign technicians are devoted to researching on the combinations of multiple techniques that are capable of removing SO2 and mercury at the same time. A method for removing mercury substances from hot flue gas invented by Foster Wheeler Energy Corporation in the United States, in which mercury in the hot flue gas is oxidized into HgCl2 by injecting a chloride salt solution into the gas flue, it can realize simultaneous removal of NOx and SO2 in combination with flue gas denitration and wet sulfur removal technologies. However, this technology is unstable in zerovalent mercury oxidization efficiency, and so far, has not yet been reported in industrialized applications.
A new flue gas purification technology invented by Ebara Corporation in Japan in which an electron beam irradiation method is employed, is featured by causing N2, O2, and H2O in the flue gas to produce large amounts of free radicals through the use of energy from high-energy electron beams to oxidize SO2 and zerovalent mercury in the flue gas, and also by generating ammonium sulfate by reaction of ammonia with an acid and recycling the reaction products. But it is difficult for this technology to be industrially applied at present since large radiological protection facilities are necessitated, operating cost is high, and energy consumption and maintenance work are great.
In China, such a technology is being developed that O3 produced by an electrostatic precipitator is utilized to oxidize zerovalent mercury in flue gas into soluble divalent mercury, combined control of multiple pollutants of denitration, sulfur removal and mercury removal can be realized in combination with the wet sulfur removal technology. Currently, this technology is still under development, and further research on the stability of mercury removal efficiency is also needed.
The technology for multi-pollutant control by adding metal chelates in wet flue gas desulfurization (FGD), which is researched and developed by American Dravo, can realize removal of sulfur and mercury by adding the metal chelates into traditional wet FGD solutions, and can achieve regeneration of absorbents only through reduction of trivalent ferric-chelates. However, with complex regeneration process of absorbents, high operating cost and poor removal efficiency of zerovalent mercury vapor, this process has not yet realized industrial application, either.
Thus, it has become an important issue in the current atmospheric pollution control field how to make full use of the existing pollutant control technologies to research and develop a practical technology of multi-pollutant control suitable for national conditions in China.